In recent years, compound semiconductor devices with high dielectric strength and high power have been intensively developed, in which the characteristics of nitride compound semiconductors, such as exhibiting high saturated electron velocity and a broad band gap, are utilized.
For example, field effect transistors such as a high electron mobility transistor (HEMT) have been developed. In particular, a GaN HEMT having an AlGaN layer being an electron supply layer is attracting attention.
In the GaN HEMT, difference in a lattice constant between AlGaN and GaN causes strain in the AlGaN layer, and the strain induces piezoelectric polarization, which generates high-concentration two dimensional electron gas in the vicinity of the upper surface of the GaN layer underlying the AlGaN layer. Such a phenomenon enables generation of high power.
The high-concentration two dimensional electron gas, however, becomes problematic in operation of normally-off transistors. A wide variety of techniques have been studied to overcome this problem. For example, techniques have been proposed, in which a p-type GaN layer doped with Mg being a p-type dopant is provided to reduce the two dimensional electron gas. The p-type GaN layer is provided between an electron supply layer and a gate electrode in some techniques or between a buffer layer and an electron transit layer in the other techniques.
Unfortunately, related art GaN HEMTs having p-type GaN layers do not have proper device characteristics in some cases while operating in a normally-off mode.
Examples of related art compound semiconductor devices are disclosed in Japanese Laid-open Patent Publication Nos. 2009-206123 and 2011-82415.